1. Field of the Invention
The present invention relates to a digital binarization circuit for converting image data from an image sensor in a facsimile apparatus or the like into digital binary numbers.
2. Description of the Prior Art
FIG. 1 is a block diagram illustrating a digital binarization circuit for image data in fixed slice system in the prior art. In FIG. 1, numeral 1 designates an image sensor which reads an image and produces an analog signal representative thereof, numeral 2 designates an analog to digital video signal processor which converts the analog signal from the image sensor 1 into a digital signal of n bits per pixel unit, numeral 3 designates a density changing circuit which outputs a changing signal of m bits so as to vary a threshold level, numeral 4 designates a threshold generator which generates a threshold value of n bits based on the changing signal of m bits outputted from the density changing circuit 3, and numeral 5 designates an n bit comparator which compares the n bit video signal from the video signal processor 2 with the threshold value of n bits and outputs a binary signal corresponding to the result of the comparison.
FIG. 2 illustrates a circuit arrangement of density changing circuit 3 and threshold generator 4. An m-bit signal representing the density of the entire document is developed by circuit 3 via a plurality of switches (electronic or the like), and is inputted to generator 4 which develops an n-bit threshold signal in response to the m-bit input signal. The threshold generator is constituted by a plurality of nodes selectively grounded or connected to a voltage source as conventionally known. In the figure, the first and second bits are fixed at "1", the n-1.sup.th and n.sup.th bits are fixed at "0", and the remaining bits are varied in response to the m-bit signal.
Next, operation will be described. An image read by the image sensor 1 is represented as an output analog signal having voltage levels corresponding to density of the picture image and is inputted to the video signal processor 2. The analog signal is subjected to signal processing such as sample/hold, A-D conversion or the like in the video signal processor 2 and converted into digital numbers of n bits per pixel.
The digital value of n bits and the threshold value of n bits from threshold generator 4 are inputted respectively to the comparator 5 and compared in value. As a result of the comparison, if the digital value of the n bits of the video signal is larger, the comparator 5 outputs H level, and if not it outputs L level. Thus, image data above the threshold level is stored as a binary number, while image data below the threshold level is stored as a zero level.
Since the digital binarization circuit of image data in the prior art is formed as above described, the operator must vary the switch setting of the threshold value to correspond to the original document If the setting is not suitable, when part of the document except for characters or the like (the background of the original document) is dark, noise may be superposed on the transmitted video signal, and when the characters in the original document are light, the characters do not appear clearly. Also under such special conditions of the original document such that differences of density of the background exist or that the dark parts and the light parts are mixed in a character, setting of the optimum threshold value is difficult.